Simplified lightmap UVs

To light a mesh efficiently using lightmaps, Enlighten produces a simplified lightmap UV mapping that minimizes the number of pixels required in the lightmap.

Efficient lightmap UVs

Enlighten groups adjacent triangles in the input mesh that form a smooth surface into charts. If two charts are placed too close together in UV space, they might refer to the same lightmap pixel. To prevent this Enlighten moves and scales charts to ensure at least one half pixel's width of unused padding space.

When a mesh has many charts that are small relative to the size of a lightmap pixel, the padding around each chart wastes a lot of lightmap space. Enlighten's UV generation tools can produce efficient UV mappings by intelligently merging charts together.

The images below show how merging charts can produce a much smaller lightmap. These efficient UV mappings are known as simplified UVs.

Nearby charts that have very similar lighting can be merged without noticeable loss of lighting accuracy.

Two adjacent charts which share an edge can often be merged into a single chart. It's necessary to preserve the boundary between charts only where a hard edge in the indirect lighting is required, such as where two surfaces meet at 90 degrees.

A smaller chart that overlaps a larger chart can be merged with the larger chart. The image below shows two versions of the same scene; in the version on the left, the near parts are merged into the same chart as the wall. 

The lightmap UV pipeline

Enlighten automatically generates lightmap UVs based on the input mesh. The pipeline can produce good results for most meshes without user intervention. The automated tools have some limitations, so the ideal pipeline allows the artist some manual control.

In practice, your mesh will already include some kind of UV mapping. Most meshes have a UV mapping used for materials, and in some scenarios there is a set of lightmap UVs used for baked lighting. In many cases, Enlighten can produce good lightmap UVs by using these mappings as a starting point.

Enlighten can detect when there is no existing UV mapping, or when using the existing UV mapping will give a poor result, and instead generates lightmap UVs from scratch using automatic UV simplification.

Lightmap UV authoring

The automated tools generally produce simplified lightmap UVs with a reasonable trade-off between quality and cost. In the rare case where the automated result is poor, and the mesh can still be efficiently lit using lightmaps, the artist can provide a manually simplified UV mapping as input.

When creating a simplified UV mapping, the artist takes control over which parts of the mesh are merged into a single chart. The image below shows the 3ds Max Unwrap UVW modifier. The boundaries between charts in the selected UV channel are highlighted on the mesh in green.

When a manually simplified UV mapping is available, configure Enlighten to ignore vertex positions and normals and consider only the existing UV mapping when forming charts. In this scenario, two adjacent faces are assigned to different charts only if they share no vertex with the same UV value, as in the layout above.